Given this strong evidence against a symmetry-protected, two-component
order parameter, the notion of an accidental (near)-degeneracy of two single-component
order parameters seems a very compelling approach [1]}, [2]}, [3]}, [4]}, [5]}.
This is rooted in significant microscopic evidence that electronic
pairing mechanisms give rise to several competing superconducting
states [6]}, [7]}, [1]}, [2]}.
A range of arguments is brought forward in Ref. [3]} supporting the degeneracy between \(\psi _{d}=d_{x^{2}-y^{2}}\) 
and \(\psi _{g}=g_{xy(x^{2}-y^{2})}\)  pairing. This state is further
supported by the observed jump of the elastic constant
in the \(B_{2g}\)  symmetry channel [11]}, [12]}, see also Ref. [13]}, and by the report of vertical line nodes [14]}.
Other candidates include the near degeneracy between
\(\psi _{d}=d_{x^{2}-y^{2}}\)  and a nodal \(s\) -wave state \(\psi _{s}\)  [2]} or the degeneracy between singlet and triplet states in a model that focuses on the quasi one-dimensional parts of the electronic structure [5]}.
Notice, an implication of the small heat-capacity signature at the lower transition is that the amplitude of this second component must be exceedingly small; see appendix 
